The present invention relates to a Cu plated ceramic substrate, a Pertier element to which the substrate is applied, and a method of manufacturing the substrate.
Cu plated materials are widely used in industrial fields. For example, in Cu plating solution, CuCN plating solution and copper pyrophosphate plating solution, a direct current is applied between Cu as anode and a workpiece as cathode to precipitate Cu on the workpiece, or Cu precipitated workpiece is made using electroless Cu plating solution.
The workpiece made of ceramic substrate is called Cu plated ceramic substrate, and the workpiece made of plastics is called a Cu plated plastic substrate.
Cu plated ceramic substrates are used as electronic parts such as semiconductor devices, small communication devices, relays, chip carriers and Pertier elements.
The surface of ceramics is treated with acid, sand blasts, etc. and is subjected to electroless Cu plating. Thereafter, Cu electroplating is applied. Alternatively, after another metal is applied on ceramic substrates with dry thin film process such as vapor deposition, Cu plating is applied. As another metal, Co, Nb, Ta, Ti, Cuxe2x80x94Nixe2x80x94Cr alloy, Mo or W is used.
The following are known to manufacture a Cu plated ceramic substrate:
{circle around (1)} Cu film or plate is directly joined on a ceramic substrate as described in J. Am. Ceram. Soc. 61, 1978 by A. K. Varchneya and R. J. Petti.
{circle around (2)} Paste of conductive material such as Ag, Au, Cu and Ni is applied on a ceramic substrate by printing or other methods, and plating is applied thereon, or after firing at several hundred degree, plating is applied.
However, diameter of Cu particle precipitated by electroless copper plating is large, so that contact area between the Cu particle and irregular portions of the surface of the ceramic substrate is small. Anchoring effect is poor, and adhesibility between the Cu particle and the ceramic substrate is low.
Furthermore, the precipitated particles are large, and a recess becomes a space, to which treating chemicals are attached to decrease adhesibility.
In the latter, conductive paste is employed, and a diameter of a particle is as large as electroless Cu plating by twenty to thirty times, so that contact area becomes very small between the particle of the conductive material and ceramic substrate to lead poor anchoring effect and decreasing adhesibility.
A Pertier element comprises a module using Pertier effect which is exothermic or endothermic phenomenon except Joule heat at a contacting point of different conductors or semi-conductors when an electric current is applied to the point and which was found by J. C. A. Pertier in 1834.
The Pertier element is employed as a cooler for a semiconductor laser device used for optical communication. The semiconductor laser converts an optical signal by a wave length converter to a shape to which WDM (Wavelength Division Multiplexing) can be made to transmit it via an optical fiber
The signal in which wavelength division is multiplexed passes through the optical fiber, and is converted to the original optical signal by a wave length converter. A number of signals can be treated as a single signal and thus transmitted by a single optical fiber, thereby reducing the number of the optical fibers significantly.
The Pertier element is employed to cool a semiconductor laser device for optical communication. The narrower the range of w temperature for controlling wavelength of a signal for converting WDM becomes, the smaller a semiconductor device becomes. Thus, the Pertier element used therein becomes smaller, and it is difficult to make such an element by attaching known copper plates. Adhesive strength of the Pertier element made by such attachment needs a larger total area. Miniaturization of the element is limited.
To overcome such problem, plating must be used with photo-forming. With miniaturization, adhesive area between the ceramic substrate and conductor circuit significantly decreases, and it is very important to improve adhesive strength between them.
The Cu plated ceramic substrate according to the present invention is suitably used as substrate for the Pertier element for cooling a miniaturized semiconductor laser.
There are two known types of Pertier elements, one being a Cu plate or film which is punched by a press and bonded to a ceramic substrate, the other being a conductive paste which is applied to a ceramic substrate, a Cu plating being applied after firing of the substrate.
However, there are disadvantages in the known Pertier substrates. In the former, miniaturization is impossible, and organic compounds must be inserted as adhesive to lead low thermal conductivity and high consumption power which is expensive. In the latter, owing to particle diameter of the conductive paste, it provides low bonding strength with the ceramic substrate and low reliability. In both elements, owing to insertion of organic compounds, the compounds deteriorate to decrease bonding strength.
In view of the disadvantages in the prior art, it is an object of the present invention to provide a Cu plated ceramic substrate to increase adhesibility between a ceramic substrate and a Cu plating layer.
To achieve the object, according to one aspect of the present invention, there is provided a Cu plated ceramic substrate which comprises:
a ceramic substrate layer;
a dry thin-film Cr layer on the ceramic substrate layer;
a dry thin-film Au layer on the thin-film Cr layer; and
a Cu plating layer on the thin-film Au layer.
According to another aspect of the present invention, there is provided a method of manufacturing a Cu plated ceramic substrate, comprising the steps of:
forming a dry thin-film Cr layer on a ceramic substrate layer;
forming a dry thin-film Au layer on the thin-film Cr layer; and
plating the thin-film Au layer with Cu.